The use and configuration of bleed valves are well known in gas turbine engines and are usually used to improve engine operability particularly for the engine's compressors. In use heated air at high pressure passes from a compressor, through a bleed valve and via a diffuser into a main gas stream, such as the relatively cool bypass flow. The compressor may be either an intermediate or high pressure compressor and exhausted bled gas temperatures may be up to 400° C.
Known bleed valve diffusers, such as installed on the Trent 500 aeroengine of Rolls-Royce™ plc, are designed with the intent to a) attenuate noise produced within the bleed valve; b) produce small separate jets of bleed air (rather than one large one) to increase the jets noise frequency, which is better attenuated within the bypass duct and atmosphere; and c) improve mixing of the hot gases flowing through the bleed valve with the cold bypass flow in order to limit/prevent thermal damage to nacelle and other components.
These diffusers are usually circular domes and have an array of holes where each hole is angled radially so that its air flow jet has both radial and axial velocity components relative to the diffuser's centre-line. Some diffusers, like the Trent 500's, have a number of zones of holes where each zone's holes are angled differently from other zones in the axial direction relative to the centre-line. This difference in axial angle between zones is intended to enhance the mixing of the hot gas passing through the diffuser with the cool bypass flow to reduce its temperature before impinging on thermally sensitive nacelle and/or engine parts. However, it has been found that these prior art diffusers are ineffective because the individual flows from each small hole do not act independently. Although each small gas stream is initially discrete and is directed in a slightly different direction, within a short distance they conglomerate into a single plume. This is because the bypass flow is unable to achieve significant penetration into the mass of individual plumes and so a partial vacuum forms between them. This vacuum causes the plumes to turn until they are pointing in the same direction and thus forming a single plume entity.
Increasing the number of bleed valve assemblies could improve mixing overall, however, this adds significant weight, cost and takes up more space. Increasing the size of the bleed valve and diffuser reduces the velocity of the same amount of bleed flow, but the larger bleed flow plume is less well mixed with the bypass stream and more likely to impinge on heat sensitive components.